水戸 毅

We have carried out 27Al nuclear magnetic resonance (NMR) measurements on YbAl3C3. The obtained NMR line is well decomposed into two powder pattern spectra characterized by the nuclear magnetic resonance (NQR) frequency νQ = 3.55MHz (Al 2d-site) and 0.35MHz (Al4f-site). About the transition at exceptionally high temperature T* = 80K, it was found that the NMR line becomes broadened rapidly just below T*, and T* increases by about 10K at the field of 30 Tesla. Analogous phenomena are commonly seen in many quadrupolar ordering systems, implying that the transition in YbAl3C3 is associated with the quadrupolar order. ©2008 The Physical Society of Japan.

We have investigated the newly discovered pressure-induced magnetic ordering phase in YbIn Cu4 using dc magnetization, nuclear magnetic resonance (NMR), and x-ray diffraction measurements. The dc magnetization measurements provide evidence for the appearance of spontaneous magnetization above 3 GPa and enable us to establish a pressure-temperature phase diagram for YbIn Cu4 over a wide pressure range up to 27 GPa. It is shown that, while the valence transition temperature TV is strongly reduced with increasing pressure, the magnetic ordering temperature TM reveals a very weak pressure dependence. We find no evidence for any structural phase transition up to 15 GPa within the experimental accuracy. The Curie-Weiss temperature dependence of susceptibility in pressure-stabilized paramagnetic state is evaluated from the Knight shift data obtained from In115 NMR measurements. From these experimental results, we discuss possible electronic states of the 4f electrons in the high pressure magnetically ordered phase of YbIn Cu4. © 2007 The American Physical Society.

We report on the discovery of exotic superconductivity (SC) and novel magnetism in heavy-fermion (HF) compounds, CeCu2Si2, CeRhIn5 and CeIn3, on the verge of antiferromagnetism (AFM) through nuclear-quadrupole-resonance (NQR) measurements under pressure (P). The exotic SC in a homogeneous CeCu2Si2 (T c ≤ 0.7K) revealed antiferromagnetic critical fluctuations at the border to AFM or a marginal AFM. Remarkably, it has been found that the application of magnetic field induces a spin-density-wave (SDW) transition by suppressing the SC near the upper critical field. Furthermore, the uniform mixed phase of SC and AFM in CeCu2(Si1-xGex) 2 emerges on a microscopic level, once a tiny amount of 1%Ge (x ≤ 0.01) is substituted for Si to expand its lattice. The application of minute pressure (P∼0.19GPa) suppresses the sudden emergence of the AFM caused by doping Ge. The persistence of the low-lying magnetic excitations at temperatures lower than Tc and TN is ascribed to the uniform mixed phase of SC and AFM. Likewise, the P-induced HF superconductor CeRhIn 5 coexists with AFM on a microscopic level in P ≤ 1.5-1.9GPa. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below the onset temperature, presumably associated with an amplitude fluctuation of superconducting order parameter. The unconventional gapless nature of SC in the low-lying excitation spectrum emerges due to the uniform mixed phase of AFM and SC. By contrast, in CeIn3, the P-induced phase separation of AFM and paramagnetism (PM) takes place without any trace for a quantum phase transition. The outstanding finding is that SC sets in at both the phases magnetically separated into AFM and PM in P ≤ 2.28-2.5GPa. A new type of SC forms the uniform mixed phase with AFM and the HF SC occurs in PM. We propose that the magnetic excitations such as spin-density fluctuations induced by the first-order phase transition from AFM to PM might mediate attractive interaction to form the Cooper pairs in the novel phase of AFM. © IOP Publishing Ltd.

In order to investigate pressure-induced magnetically ordered phase in YbInCu4, the DC magnetization has been measured up to 7.57 GPa by using the combined system of a diamond-anvil pressure cell and a superconducting quantum interference device (SQUID) with a vibrating coil magnetometer. In the pressure range above 3.05 GPa, a rapid increase in the magnetization below TM = 2.5 K was observed, giving the first direct evidence of spontaneous magnetization in the high pressure phase of YbInCu4. TM shows weak pressure dependence up to the maximum pressure of 7.57 GPa. A phase diagram for YbInCu4 over a wide range of pressure is presented. © 2006 Elsevier B.V. All rights reserved.

In order to investigate the microscopic origin of the transitions at T-MI similar to 16.5K and T* similar to 14K in filled skutterudite compound SmRu4P12, we have carried out P-31-NMR measurements using a single crystal sample. The drastic change in the NMR line-shape below TMI was observed, which is attributed to the occurrence of some long-range magnetic ordering. The order is probably associated with octupole ordering as indicated from the NMR measurements on the powder sample of SmRu4P12. (c) 2006 Elsevier B.V. All rights reserved.

We report the novel phase diagram of superconductivity (SC) and antiferromagnetism (AFM) in heavy-fermion (HF) pressure(P)-induced superconductors CeRhIn5 and CeIn5 through nuclear-quadrupole-resonance (NQR) measurements under P. The P-induced superconductivity in CeRhIn5 coexists with AFM on a microscopic level in P=1.5-1.9 GPa. By contrast, in CeIn3, the P-induced phase separation of AFM and paramagnetism (PM) takes place without any trace of a quantum phase transition. The outstanding finding is that SC sets in at both the phases magnetically separated into AFM and PM in P=2.28-2.5 GPa. © 2005 Elsevier Ltd. All rights reserved.

We review the current NMR/NQR studies on superconducting and magnetic phases close to the magnetic instability under pressure (P) in heavy-fermion (HF) systems. In this article, by means of NQR studies, we highlight the novel phase diagrams of magnetism and superconductivity (SC) in the HF antiferromagnets CeCu2Si2, CeRhIn5, CeIn 3, and the itinerant ferromagnet UGe2. A new light is shed on the difference and common features on the interplay between magnetism and SC. © 2005 Elsevier B.V. All rights reserved.

Applying pressure to the valence fluctuating compound YbInCu4 lowers the valence transition temperature and stabilizes the high temperature (HT) localized spin state. For pressure above 2.4 GPa, the valence transition is completely suppressed and the magnetically ordered state arises as a new ground state below 2.4 K. In order to elucidate microscopically the relation between the valence and magnetic transitions in f electron system, we performed 63Cu NQR measurements of YbInCu4 under pressure up to 2.5 GPa. The experimental results provide clear evidence for the onset of the magnetic ordering in the pressure-stabilized HT state above 2.3 GPa. The nuclear spin-lattice relaxation rate 1/T1 in the HT state shows 1/T 1 = constant behaviour down to 3.0 K and a divergence increase just below 2.4 K. We concluded that the pressure-stabilized localized electron state of YbInCu4 transforms directly into a magnetically ordered state, without displaying any intermediate heavy Fermi liquid behaviour. © 2005 IOP Publishing Ltd.

We report on the pressure (P) versus temperature (T) phase diagram of the P-induced heavy fermion (HF) superconductor CeRhIn5 via nuclear quadrupole resonance (NQR) measurements under P. At P = 2.1 GPa, the T dependence of the nuclear spin-lattice relaxation rate 1/T1 revealed antiferromagnetic critical fluctuations due to the closeness of antiferromagnetism. As P decreases slightly down to 1.9 GPa, a pseudogap emerges above Tc ∼ 2 K, suggesting the appearance of superconducting fluctuations associated with a strong coupling effect among quasiparticles. © 2005 IOP Publishing Ltd.

To elucidate microscopically the effect of pressure on the physical property of YbIn Cu4, we have carried out Cu63 nuclear quadrupole resonance and In115 nuclear magnetic resonance measurements under high pressure. Above 2.4 GPa, the pressure-stabilized localized f electron phase turns out to transform directly into the magnetically ordered state with nearly a full moment of Yb3+, without displaying any intermediate heavy Fermi-liquid behavior. We propose a schematic T-D (εF) Jcf phase diagram to help in understanding the physics on the phase boundary curves between each of the three states: the localized f electron, valence fluctuating, and magnetically ordered states. © 2005 The American Physical Society.

We review the studies on the emergent phases of superconductvity and magnetism in the f-electron derived heavy-fermion (HF) systems by means of the nuclear-quadrupole-resonance (NQR) under pressure. These studies have unraveled a rich variety of the phenomena in the ground state of HF systems. In this article, we highlight the novel phase diagrams of magnetism and unconventional superconductivity (SC) in CeCu 2Si 2, HF antiferromagnets CeRhIn 5, and CeIn 3. A new light is shed on the difference and common features on the interplay between magnetism and SC on the magnetic criticality. ©2005 The Physical Society of Japan.

Formation of the antiferroquadrupolar (AFQ) ordering in YbSb (NaCl-type structure) was evidenced microscopically with the 121Sb and 123Sb nuclear magnetic resonance (NMR) measurements. We have observed in the NMR line width below TQ=5 K the appearance of an additional field-independent contribution that is proportional to the nuclear electric-quadrupole moment. Thus, the field-independent contribution can be addressed not to any long-range magnetic orderings but to a broken symmetry of the charge distribution associated with the AFQ ordering. The AFQ ordering also gives rise to a large anisotropic Knight shift broadening caused by the anisotropy in the transferred hyperfine field and/or magnetic susceptibility. Fluctuations of the Yb 4f moments are also investigated by using the 121,123Sb spin-lattice relaxation rate measurements.

We report on experimental studies of the phase state and the character of phase transitions in the quasi-one-dimensional organic compound (TMTSF) 2PF6 in the close vicinity of the borders between the metal (M), antiferromagnetic (AF) insulator, and superconducting (SC) states. In order to drive the system precisely through the phase border P 0(T0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for smooth and precise tuning δP = P - P0 [thanks to a monotonic P0(B) dependence] and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured magnetoresistance R(B) and its temperature dependence R(B,T) in the pressure range 0-1 GPa. Our studies focus on the features of the magnetoresistance at the phase transitions between the M and AF phases and in the close vicinity to the superconducting transition at T ≈ 1 K. We found pronounced history effects arising when the AF/M phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the M to AF phase, driven by increasing magnetic field, the features of the M phase extend well into the AF phase. At the opposite transition from the AF to M phase, the features of the AF phase are observed in the M phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase, spatially separated from the majority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the phase boundary.

We report on experimental studies of the phase state and the character of phase transitions in the quasi-one-dimensional organic compound (TMTSF)(2)PF6 in the close vicinity of the borders between the metal (M), antiferromagnetic (AF) insulator, and superconducting (SC) states. In order to drive the system precisely through the phase border P-0(T-0), the sample was maintained at fixed temperature T and pressure P, whereas the critical pressure P-0 was tuned by applying the magnetic field B. In this approach, the magnetic field was used (i) for smooth and precise tuning deltaP=P-P-0 [thanks to a monotonic P-0(B) dependence] and (ii) for identifying the phase composition (due to qualitatively different magnetoresistance behavior in different phases). Experimentally, we measured magnetoresistance R(B) and its temperature dependence R(B,T) in the pressure range 0-1 GPa. Our studies focus on the features of the magnetoresistance at the phase transitions between the M and AF phases and in the close vicinity to the superconducting transition at Tapproximate to1 K. We found pronounced history effects arising when the AF/M phase border is crossed by sweeping the magnetic field: the resistance depends on a trajectory which the system arrives at a given point of the P-B-T phase space. In the transition from the M to AF phase, driven by increasing magnetic field, the features of the M phase extend well into the AF phase. At the opposite transition from the AF to M phase, the features of the AF phase are observed in the M phase. These results evidence for a macroscopically inhomogeneous state, which contains macroscopic inclusions of the minority phase, spatially separated from the majority phase. When the system is driven away from the transition, the homogeneous state is restored; upon a return motion to the phase boundary, no signatures of the minority phase are observed up to the phase boundary.

To elucidate the combined magnetic behavior of BaCuO2+x- (x = 0, 0.14), which contains in the BCC unit-cell two Cu18O24 spherical clusters, eight Cu6O12 ring clusters and six CuO4 lone units, we have carried out the magnetization, 63Cu NMR, and 63Cu PQR measurements. Quantitative analysis of present susceptibility data indicates that the spins within a Cu6 cluster of BaCuO2.00 are in the ferromagnetically ordered ground state with maximum spin Sr = 3 even at room-T, and the spins within a Cu18 cluster are in a ground state below ∼20K with net spin Ss∼5 less than the maximum spin. 63Cu NMR for BaCuO2.00 was observed below ∼15K with a trapezoid-like shape spectrum and the relaxation rate T1-1 was roughly proportional to T3. These are consistent with the long-range antiferromagnetic ordering of the Cu6 cluster-spins below T N = 15 K. The NMR spectrum for BaCuO2.14 has an electric-quadrupole split structure down to 1.7 K, which is indicative of no long-range magnetic ordering. We found, however, a few anomalies in the NMR and PQR data for BaCuO2.14, which could be understood by a short-range ordering of paramagnetic Cu18 cluster-spins. © 2003 Published by Elsevier B.V.

Application of pressure on the valence fluctuating compound YbInCu 4 suppresses the valence transition temperature and stabilizes the high-temperature (HT) magnetic phase. We have investigated microscopically the pressure effect on the electronic states of YbInCu4 with Cu-NQR under up to 1.68 GPa. The effect of pressure appears in the increase of quadrupole frequencies mainly caused by shrinking of the cell volume. Whereas the temperature-independent behavior of spin-lattice relaxation time T1 in the HT phase and T1T= constant metallic behaviour in the LT phase are hardly affected by pressure, indicating that the electronic states in both HT and LT phases are rather insensitive to pressure up to 1.68 GPa. © 2003 Elsevier B.V. All rights reserved.

We report similarities and differences of the transport features in the spin density wave (SDW) and in the field-induced SDW (FISDW) phases of the quasi-one-dimensional compound (TMTSF)"2PF"6. As temperature decreases below ≈2 K, the resistance in both phases exhibits a maximum and a subsequent strong drop. However, the characteristic temperature of the R(T) maximum and its scaling behavior in different magnetic fields B are evidence that the nonmonotonic R(T) dependences have different origin in SDW and FISDW regions of the phase diagram. We also found that the borderline T"0(B, P) which divides the FISDW region of the P-B-T phase diagram into the hysteresis and nonhysteresis domains terminates in the N = 1 subphase; the borderline thus has no extension to the SDW N = 0 phase. © 2004 MAIK "Nauka/Interperiodica".

We report a pressure (P) effect on CeCu2(Si 0.98Ge0.02)2 where an antiferromagnetic (AFM) order at TN ∼ 0.75 K coexists with superconductivity below T c ∼ 0.4 K. At pressures exceeding P = 0.19 GPa, the AFM order is suppressed, which demonstrates that the sudden emergence of AFM order due to the Ge doping is ascribed to the intrinsic lattice expansion. The exotic superconductivity at P = 0 GPa is found to evolve into a typical heavy-fermion one with a line-node gap above P = 0.91 GPa. We highlight that the anomalous enhancement in nuclear spin-lattice relaxation rate 1/T1 that follows a T1T = const. behavior well below Tc at P = 0 GPa is characterized by the persistence of low-lying magnetic excitations, which may be inherent to the coexistent state of antiferromagnetism and superconductivity. ©2004 The Physical Society of Japan.

To clarify the combined magnetic behaviour of BaCuO2+x, which contains in the body-centred-cubic unit cell two Cu18O24 spherical clusters, eight Cu6O12 ring clusters, and six CuO4 lone units, we made measurements of magnetization, 63,65Cu nuclear magnetic resonance (NMR) near 6 T, and pure quadrupole resonance (PQR) in zero field on samples with well-defined oxygen contents x = 0 and 0.14. Quantitative analyses of Curie-Weiss-like susceptibility data for BaCuO2.00 indicate that Cu spins within the Cu6 clusters go into a ferromagnetically (FM) ordered ground state with a maximum spin Sr = 3 below ∼100 K, and those within the Cu18 clusters into an FM-like ground state below ∼20 K with net spin Ss ∼ 5 that is less than the maximum possible spin Ss = 9. For BaCuO2.14 the Cu spins within the Cu6 clusters go into an FM ordered ground state at a lower temperature of ∼30 K, and these clusters exhibit no signature of long-range magnetic ordering down to 2 K. The NMR spectra for BaCuO2.00 observed below ∼12 K have a trapezoidal structure that is known to be a characteristic of long-range magnetic ordering, and is consistent with the antiferromagnetic ordering of the Cu6 clusters below TN = 15 K predicted previously. On the other hand, the spectrum for BaCuO2.14 observed in a temperature range between 1.2 and ∼50 K has an electric quadrupole-split powder pattern structure. This provides microscopic evidence that there is no long-range magnetic ordering in BaCuO2.14. The addition of 0.14 moles of oxygen atoms results in a large suppression of some magnetic entities. We find, however, some anomalies in the NMR and PQR data for BaCuO2.14 near ∼10 K, which could be associated with short-range ordering of the paramagnetic Cu18 clusters.

The systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum and nuclear-spin-lattice-relaxation time (T1) on the pressure (P)-induced heavy-fermion superconductor CeRhIn5 were reported. The temperature (T) dependence of 1/T 1 at P = 1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at TN=2.8 K and TcMF = 0.9 K. It was demonstrated that SC does not yield and trace of gap opening in low-lying excitations below Tconset = 2 K.

We report In-NQR and Co-NMR experiments of CeCoIn5 that undergoes a superconducting transition with a record high Tc = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, an anomalous temperature (T) dependence of nuclear spin-lattice relaxation rate 1/T1 of 115In is explained by the relation 1/T1 ∝ T · χQ(T)3/4 based on the anisotropic spin-fluctuations model in case of the proximity to an antiferromagnetic (AFM) quantum critical point (QCP). The novel behavior of 1/T1 ∼ T 1/4 over a wide T range of Tc < T < 40 K arises because the staggered susceptibility almost follows the Curie law χQ(T) ∝ 1/(T + θ) with θ = 0.6 K and hence 1/T1 ∝ T/(T + 0.6)3/4 ∼ T1/4 for θ < T. We highlight that the behavior 1/T1 ∼ T 1/4 is due to the proximity to the anisotropic AFM QCP relevant with its layered structure, and is not associated with the AFM QCP for isotropic 3D systems. We have also found that the AFM spin fluctuations in CeCoIn5 are suppressed by small magnetic field so that θ = 0.6 K at H = 0 increases to θ = 2.5 K at H = 1.1 T, reinforcing that CeCoIn5 is closely located at the QCP. © 2003 The Physical Society of Japan.

We have investigated physical properties of the valence fluctuating compound YbInCu4 at high pressures and low temperatures. The first-order valence transition temperature TV ∼ 42 K at ambient pressure is completely suppressed for pressures above 2.49 GPa. Present electrical resistivity data in the pressure- stabilized high-temperature (HT) phase exhibit successive shoulders at around 20 and 2.4 K. The former can reasonably be compared with the Kondo temperature TK ∼ 25 K estimated from the static-susceptibility data at ambient pressure. For pressures above 2.27 GPa, the ac-susceptibility showed a clear peak at 2.4 K, which can be explained as the onset of a long-range ferromagnetic ordering. 63Cu pure-quadrupole-resonance in the HT phase showed that the value of spin-lattice relaxation time T1 and the almost T-independent behavior are hardly affected with pressure up to 1.3 GPa, in contrast to the large increase in quadrupole frequency.

To elucidate the ground state of the pressure-stabilized high-temperature (HT) phase of YbInCu4, we have carried out electrical resistivity ρ and ac-susceptibility χac measurements at high pressures. For pressures above 2.49 GPa, the first-order valence transition is completely suppressed (below 80 mK). Separately, above 2.39 GPa, a clear peak appears in χac with a small kink in ρ at around TM = 2.4 K. The χac peak is easily diminished by applying low magnetic fields and disappears above 500 Oe. The characteristic behavior of χac at TM can generally be ascribed to the onset of long-range ferromagnetic ordering and, therefore, the ground state of the pressure-stabilized HT phase is most probably a ferromagnetically ordered state. This result is compatible with the occurrence of weak ferromagnetism recently reported for the Y-substituted compound Yb0.8Y0.2InCu4 under pressure of 1.2 GPa. © 2003 The American Physical Society.

Nuclear quadrupolar resonance measurements find that, under hydrostatic or chemical pressure, the heavy fermion antiferromagnet CeRhIn5 becomes more itinerant and the Neel temperature TN increases slightly at low pressures but decreases at large pressures. In CeRh0.5Ir0.5In 5, unconventional superconductivity sets in well below T N, with enhanced Tc ≈ 1 K. © 2003 Elsevier Science B.V. All rights reserved.

We report evidences for bulk nature and unconventional superconductivity in CeIn3 at P = 2.65 GPa revealed by the measurements of nuclear-spin-lattice-relaxation time (T1) and ac-susceptibility (ac-χ). Both the measurements of ac-χ and T1 have pointed to a superconducting transition at T = 95 mK, which is very much lower than an onset temperature Tonset = 0.15 K at zero resistance. The temperature dependence of 1/T1 shows no coherence peak just below Tc, indicative of an unconventional nature for the superconductivity. © 2003 Elsevier Science B.V. All rights reserved.

We have investigated microscopically the heavy-fermion properties of YbZnCu4 and YbAuCu4 with 63Cu NMR and PQR measurements. Both the isotropic and axial Knight shifts for each of the compounds showed a Curie-Weiss-type behavior, which is indicative of the localized Yb3+ moments. The nuclear spin-lattice relaxation rate (T1T)-1 for YbZnCu4 above 1.4 K and for YbAuCu4 above ∼ 50 K was proportional to the uniform susceptibility χ, indicating that the correlation time τ f-1 of Yb-spins is nearly independent of temperature. (T1T)-1 for YbAuCu4 below ∼ 50 K exhibited a prominent increase associated probably with the decrease in τ f-1 to the Kondo fluctuation rate τK-1 = kBTK/ℏ. © 2003 Elsevier Science B.V. All rights reserved.

Magnetoresistance measurements on the quasi one-dimensional organic conductor (TMTSF)2PF6 performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K, and under pressures P up to 1.4 GPa have revealed new phase boundary on the P-B-T phase diagram. The boundary subdivides the field induced spin density wave (FISDW) phase diagram into two regions. The low-temperature region of the FISDW diagram is characterized by a hysteresis behavior of the resistance R(B) which was observed in earlier studies and is typical for the first-order transitions. In contrast to the common believe, in the high temperature region of the FISDW phase diagram, the hysteresis in R(B) and, hence, the first-order transitions vanish. Nevertheless, sharp changes in dR/dB are observed both in the low and high temperature domains indicating that the transitions between different sub-phases exist over all range of the FISDW state. We compare these results with a theoretical prediction. © 2002 Elsevier Science B.V. All rights reserved.

By performing the 115In NQR and the ac-susceptibility measurements, it was found that AFM and SC coexist homogeneously at a microscopic level in the HF compound CeRhIn5 at P=1.75 GPa. The results also shed light on other strongly correlated systems including high-Tc.

We report the pressure (P)- induced evolution of the antiferromagnetism in CeRhIn5 which undergoes a superconducting transition at T-c similar to 2.1 K at pressurs exceeding P(c)similar to 1.6 GPa. Rom measurements of In-115 nuclear-spin-lattice-relaxation time (T-1) under P, we found that Neel temperature T-N is reduced above P = 1.23 GPa, which is accompanied by an emergent pseudogap behavior.

We report systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ([Formula presented]) on the pressure ([Formula presented])-induced heavy-fermion superconductor [Formula presented]. The temperature ([Formula presented]) dependence of [Formula presented] at [Formula presented] has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at [Formula presented] and [Formula presented]. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below [Formula presented], but [Formula presented], followed by a [Formula presented] law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in the low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below [Formula presented]. © 2003 The American Physical Society.

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound [Formula presented] under pressure [Formula presented]. The onset of the magnetic order is evidenced from a clear split of [Formula presented] nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Néel temperature [Formula presented]. Simultaneously, bulk SC below [Formula presented] is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level. © 2003 The American Physical Society.

Phase transitions occuring in a quasi-one-dimensional organic compound (TMTSF)2PF6 near the boundaries between the paramagnetic metallic (PM), antiferromagnetic insulator (AFI), and superconducting (SC) states were studied experimentally. A controlled transition through the phase boundary was achieved by maintaining the sample at fixed temperature T and pressure P, while the critical pressure was tuned by varying a magnetic field B. When the PM/AFI phase boundary was crossed due to the variation of a magnetic field, history effects were observed: the resistance was found to depend on the trajectory described by the system before arriving at a given point (P-B-T) of the phase space. The results of the experiment give evidence for the formation of a macroscopically inhomogeneous state characterized by the inclusions of a minor phase that is spatially separated from the major phase. Away from the phase boundary, the homogeneous state is restored. After this, upon approaching the phase boundary in the back direction, the system exhibits no features of the minor phase up to the very boundary. © 2003 MAIK "Nauka/ Interperiodica".

We have carried out a 93Nb nuclear magnetic resonance (NMR) study on a quasi-one-dimensional superconductor, Nb3Te4, and the isostructural intercalated compounds AxNb3Te4 (A = In and Zn), to understand the varied impact of metal-atom intercalations into the hexagonal infinite tunnels on the charge density wave and superconductivity formations. Below the Peierls transition temperature TC for each of the compounds, the nuclear spin-lattice relaxation rate (T1T)-1 exhibited a significant decrease which originates from the partial Fermi surface quenching associated with the Peierls transition. With x ≃ 0.1 introductions of In and Zn, (T1T)-1 was greatly enhanced from 10 s-1 K-1 to 20 and 18 s-1 K-1, respectively. On the other hand, the nuclear quadrupole frequency vQ was diminished from 0.98 MHz to 0.63 and 0.74 MHz, respectively. A remarkable difference between the In and Zn intercalations was found in the extent of decrease in the isotropic Knight shift Kiso: from 0.237 to 0.216% for In; and to 0.166% for Zn. Based upon the d band structures predicted theoretically, we have indicated that the variations in the NMR parameters can be explained consistently, if the extent of hole doping by a Zn atom is much larger than that by an In atom. The heavy hole doping by Zn intercalation changes the dominant subband at EF from dyz,zx to dx(2)-y(2), giving rise to a distinct nesting condition of the Fermi surface.

We report evidence for unconventional superconductivity in CeIn3 at a pressure P=2.65 GPa above the critical pressure (P(c)similar to2.5 GPa) revealed by measurements of the nuclear-spin-lattice-relaxation time (T-1) and ac susceptibility (ac-chi). Both measurements of T-1 and ac-chi have pointed to a superconducting transition at T-c=95 mK, which is much lower than the onset temperature T-onset=0.15 K at zero resistance. The temperature dependence of 1/T-1 shows no coherence peak just below T-c, indicative of an unconventional nature for the superconductivity induced in CeIn3.

We report evidence for unconventional superconductivity in CeIn3 at a pressure P=2.65 GPa above the critical pressure (Pc∼2.5 GPa) revealed by measurements of the nuclear-spin-lattice-relaxation time (T1) and ac susceptibility (ac-x). Both measurements of T1 and ac-x have pointed to a superconducting transition at Tc=95 mK, which is much lower than the onset temperature Tonset=0.15 K at zero resistance. The temperature dependence of 1/T1 shows no coherence peak just below Tc, indicative of an unconventional nature for the superconductivity induced in CeIn3.

We have carried out a systematic 63Cu nuclear magnetic resonance (NMR) study on a set of ytterbium-based Kondo compounds YbXCu4 with X = Au, Ag, In, Cd, Tl, and Mg. Splitting of the central NMR line due to a second-order electric-quadrupole interaction is of the order of magnitude of axial Knight shift, and the extent of splitting is controlled by changing applied field H. From the splitting of the central line, we have succeeded to deduce the values of both isotropic Knight shift Kiso and axial Knight shift Kax, taking a value of electric-quadrupole frequency determined by pure quadrupole resonance of 63Cu. Kiso versus magnetic susceptibility χ plots for each of the compounds with X = Au, Ag, and In are roughly on a straight line. For YbAgCu4 (Kondo temperature TK ∼ 100 K), both Kiso and the unit-cell volume vc reach a local minimum around 40 K. We have found a linear relation between Kiso and vc below 100 K, similar to that observed in YbInCu4, indicating that the nonmagnetic behavior at low temperatures can be ascribed mainly to the Kondo volume expansion. In contrast, Kiso versus χ plots for YbCdCu4 (TK∼220 K) and YbMgCu4 (TK∼860 K) exhibit somewhat complex behavior: hyperfine field Hhf markedly increases coincident with the saturated behavior of χ for X = Cd below ≃ 140 K, and with the decrease in χ for X = Mg below ∼260 K. Hhf originates mainly from transferred hyperfine coupling between Cu nucleus and Yb 4f moment, and the large increase in Hhf is conjectured to result from a variation of crystal-electric-field interactions as the system transforms into a mixed-valence state. The variation with the species of X atoms of temperature-independent on-site contribution Ks to the Knight shift is found to correlate with that of the electronic specific heat coefficient γ (except for X = Cd), each of which gives a measure of the density of states of conduction sf resonance bands. Finally, using the values of Ks, γ, and TK, we have proposed a phase diagram for YbXCu4 series, which corresponds to Doniach's phase diagram.

We report a pressure effect on CeCu2(Si0.98Ge0.02)2 where an antiferromagnetic order below TN∼0.75 K coexists with superconductivity below Tc∼0.4 K. At pressures exceeding 0.56 GPa, the magnetic order is suppressed and concomitantly the unconventional superconducting state evolves into a typical heavy-fermion one with a line-node gap. This result demonstrates that the magnetic order is due to the lattice expansion caused by doping Ge. We remark that the characteristic feature of exotic coexisting state at ambient pressure is the persistence of low-lying magnetic excitations even in the superconducting state. © 2002 Elsevier Science B.V. All rights reserved.

We report 115In nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field at the In site is substantially reduced from 1.75 kOe at P = 0 to 0.39 kOe at P = 1.23 GPa, while TN slightly changes with increasing P. This suggests that either the size or the direction of the ordered moment changes as P increases. In the SC state at P = 2.1 GPa, the nuclear spin-lattice relaxation rate has revealed a T3 dependence without the coherence peak just below Tc, giving evidence for the unconventional superconductivity. © 2002 Elsevier Science B.V. All rights reserved.

Magnetoresistance measurements on the quasi-one-dimensional organic conductor (TMTSF)2PF6 performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K and under pressures P up to 14 kbar have revealed new phases on its P-B-T phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. Whereas a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first-order transitions, in a hightemperature region of the FISDW phase diagram, the hysteresis was found to disappear. We also found that the temperature dependence of the resistance (at a constant B) changes sign at about the same boundary. We compare these results with recent theoretical models.

We report the pressure (P)-temperature (T) phase diagram of antiferromagnetism and superconductivity in CeRhIn5 and CeIn3 revealed by the 115In nuclear-spin-lattice-relaxation (T1) measurement. In the itinerant magnet CeRhIn5, we found that the Néel temperature TN is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. In CeIn3, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9 GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations.

We report evidence for unconventional superconductivity in (formula presented) at a pressure (formula presented) above the critical pressure (formula presented) revealed by measurements of the nuclear-spin-lattice-relaxation time (formula presented) and ac susceptibility (ac- (formula presented) Both measurements of (formula presented) and ac- (formula presented) have pointed to a superconducting transition at (formula presented) which is much lower than the onset temperature (formula presented) at zero resistance. The temperature dependence of (formula presented) shows no coherence peak just below (formula presented) indicative of an unconventional nature for the superconductivity induced in (formula presented). © 2002 The American Physical Society.

We have carried out a systematic (formula presented) nuclear magnetic resonance (NMR) study on a set of ytterbium-based Kondo compounds (formula presented) with (formula presented) Ag, In, Cd, Tl, and Mg. Splitting of the central NMR line due to a second-order electric-quadrupole interaction is of the order of magnitude of axial Knight shift, and the extent of splitting is controlled by changing applied field H. From the splitting of the central line, we have succeeded to deduce the values of both isotropic Knight shift (formula presented) and axial Knight shift (formula presented) taking a value of electric-quadrupole frequency determined by pure quadrupole resonance of (formula presented) (formula presented) versus magnetic susceptibility (formula presented) plots for each of the compounds with (formula presented) Ag, and In are roughly on a straight line. For (formula presented) (Kondo temperature (formula presented) both (formula presented) and the unit-cell volume (formula presented) reach a local minimum around 40 K. We have found a linear relation between (formula presented) and (formula presented) below 100 K, similar to that observed in (formula presented) indicating that the nonmagnetic behavior at low temperatures can be ascribed mainly to the Kondo volume expansion. In contrast, (formula presented) versus (formula presented) plots for (formula presented) (formula presented) and (formula presented) exhibit somewhat complex behavior: hyperfine field (formula presented) markedly increases coincident with the saturated behavior of (formula presented) for (formula presented) below (formula presented) and with the decrease in (formula presented) for (formula presented) below (formula presented) (formula presented) originates mainly from transferred hyperfine coupling between Cu nucleus and Yb (formula presented) moment, and the large increase in (formula presented) is conjectured to result from a variation of crystal-electric-field interactions as the system transforms into a mixed-valence state. The variation with the species of X atoms of temperature-independent on-site contribution (formula presented) to the Knight shift is found to correlate with that of the electronic specific heat coefficient (formula presented) (except for (formula presented) each of which gives a measure of the density of states of conduction (formula presented) resonance bands. Finally, using the values of (formula presented) γ, and (formula presented) we have proposed a phase diagram for (formula presented) series, which corresponds to Doniach’s phase diagram. © 2002 The American Physical Society.

We report the pressure (formula presented)-temperature (formula presented) phase diagram of antiferromagnetism and superconductivity in (formula presented) and (formula presented) revealed by the (formula presented) nuclear-spin-lattice-relaxation (formula presented) measurement. In the itinerant magnet (formula presented) we found that the Néel temperature (formula presented) is reduced at (formula presented) with an emergent pseudogap behavior. In (formula presented) the localized magnetic character is robust against the application of pressure up to (formula presented) beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. © 2001 The American Physical Society.

Magnetoresistance measurements on the quasi-one-dimensional organic conductor (formula presented) performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K and under pressures P up to 14 kbar have revealed new phases on its (formula presented) phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. Whereas a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first-order transitions, in a high-temperature region of the FISDW phase diagram, the hysteresis was found to disappear. We also found that the temperature dependence of the resistance (at a constant (formula presented) changes sign at about the same boundary. We compare these results with recent theoretical models. © 2002 The American Physical Society.

We report a novel pressure(P)-induced evolution on the magnetism in the P-induced superconductor CeRhlns revealed by measurements of the 115 In nuclear-spin-lattice-relaxation rate (1151/Ti). In the itinerant antiferromagnet CeRhln5 with the Néel temperature Tn = 3.8 K at P=0, Tn begins to be reduced to Tn = 3.6 K at P = 1.23 GPa that is close to the border of the superconducting phase. An emergent behavior is the observation of a pseudogap behavior that is revealed by a broad peak in the temperature dependence of 1/T1T at P = 1.23 GPa. With further increasing P up to P = 2.1 GPa where its bulk superconductivity is fully established, the pseudogap behavior is not seen in the normal state. The insensitive P dependence of Tc in CeRhln5 is suggested to be related to that of the characteristic temperature T which marks a localized to itinerant magnetic crossover. © 2002 1 he Physical bociety of Japan.

We report the novel pressure (P)-temperature (T) phase diagrams of antiferromagnetism (AFM) and superconductivity (SC) in CeRhIn5, CeIn3, and CeCu2Si2 revealed by the nuclear quadrupole resonance measurement. In the itinerant helical magnet CeRhIn5, we found that the Néel temperature TN is reduced at P ≥ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AFM and SC is found in a narrow P range of 1.63-1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1-5 GPa. In CeIn3, the localized magnetic character is robust against the application of pressure up to P ∼ 1.9GPa, beyond which the system evolves into an itinerant regime in which the resistive superconducting phase emerges. We discuss the relationship between the phase diagram and the magnetic fluctuations. In CeCu2Si2, the SC and AFM coexist on a microscopic level once its lattice parameter is expanded. We remark that the underlying marginal AFM state is due to collective magnetic excitations in the superconducting state in CeCu2Si2. An interplay between AFM and SC is discussed on the SO(5) scenario that unifies AFM and SC. We suggest that the SC and AFM in CeCu2Si2 have a common mechanism. © 2002 Elsevier Science Ltd. All rights reserved.